External defibrillators frequently include a pair of “hands-free” disposable electrodes, which are essentially flexible pads that are adhered to the skin of a patient having a cardiac event (i.e., used transcutaneously). By “hands-free,” we mean electrodes of the type that are adhered to a patient, rather than paddles that are held by a rescuer during defibrillation. Hands-free disposable electrodes typically include a non-conductive backing layer, a conductive metal layer, formed from a thin sheet of metal (e.g. tin or silver) or a conductive ink (e.g. silver-chloride) printed on a substrate, and a liquid or solid electrically conductive gel covering the metal layer so that electrical current passes through the gel to the patient's body. The area of contact between the gel and the patient's body where current is delivered is referred to herein as the “treatment area”. Because such electrodes use a thin sheet of metal, flexibility is limited and cracking results from repeated use. Wire mesh or expanded metal have been proposed as a solution to this problem, but wire mesh provides for extraneous “noise” in ECG monitoring and expanded metal is also prone to cracking. Metal cracking results in arcing or failure to deliver therapy as required. As a result such typical electrodes are not reusable, requiring purchase of electrodes after use and consequently, increased costs.
External defibrillators also routinely use paddle electrodes, such as disclosed in Scharnberg U.S. Pat. No. 4,779,630. These paddle electrodes are not “hands-free”. Typically, the rescuer applies a liquid gel to the metallic surface of the paddles, and presses the gelled surface against the chest of the patient during delivery of the defibrillation shock. Scharnberg also discloses an alternative construction in which disposable, gel-containing pads are secured to the metallic surface of the paddles. But the paddles with attached pads must still be held against the chest by the rescuer.
One important property of electrodes is that the material used in the metal layer depolarize quickly (within seconds) after a defibrillating pulse (“shock”) is delivered to a patient. Otherwise, the electrode is not capable of sensing a signal that will allow the defibrillator to generate a clear ECG and determine whether another shock should be delivered within a short period of time.
US Patent Application Publication No. 2008/0221631, the disclosure of which is herein incorporated by reference, discloses that stainless steel, and other metals that polarize during a defibrillating pulse, can be used as the conductive metal layer in a defibrillating electrode, provided that the defibrillator with which the electrode is used is configured to deliver to the patient a defibrillation waveform that is capable of rapidly depolarizing the electrode. Stainless steel is an advantageous material for the conductive layer, as it is resistant to corrosion, thereby providing a long electrode shelf life. Stainless steel is also strong, and thus its use in the conductive layer reduces the likelihood that the electrode will be damaged by mishandling.
Such depolarizing waveforms include biphasic waveforms, e.g., those which are discussed in detail in U.S. Pat. No. 5,769,872, the disclosure of which is incorporated herein by reference. As disclosed in the earlier application, it is believed that the negative phase of a biphasic waveform reduces or eliminates the electrical charge, allowing the electrode to rapidly depolarize after the defibrillating pulse is delivered.